Carbon monoxide is usually obtained by separation from synthesis gases produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. In addition to carbon monoxide, these gases contain primarily hydrogen and methane but are often contaminated with significant amounts of nitrogen (derived from the feed or added during processing). Conventional cryogenic separation processing leaves nitrogen as an impurity in the carbon monoxide, which, for both environmental and processing reasons, is unacceptable for some uses of carbon monoxide. The problem of nitrogen contamination of carbon monoxide product is becoming an increasing problem with the usage of more marginal feed stock in front end reforming processes. Further, there is an increasing demand for carbon monoxide to be free of argon, which usually is a co-contaminant with nitrogen. Accordingly, there is a demand for efficient and effective removal of contaminant nitrogen and, if required, argon from carbon monoxide-containing feeds.
The separation of nitrogen alone or with argon co-contaminant from carbon monoxide is relatively difficult compared to removal of hydrogen or methane. Prior art processes for removing nitrogen from synthesis gas usually include the sequential steps of removing hydrogen from the synthesis gas feed, removing methane from the resultant hydrogen-freed stream, and removing nitrogen from the resultant hydrogen- and methane-freed stream to leave a purified CO product stream.
U.S. Pat. No. 4,478,621 discloses such a process for the recovery of carbon monoxide in which synthesis gas feed is partially condensed and the resultant two phase mixture fed to a wash column in which carbon monoxide is scrubbed from the vapor phase by contact with a liquid methane stream to provide CO-loaded methane containing some, typically 3-4%, hydrogen. A CO recycle heat pump stream provides intermediate indirect cooling to the wash column to remove the heat of solution of carbon monoxide in methane. Residual hydrogen is removed from the CO-loaded methane in a stripping column to meet the required carbon monoxide product specification. The hydrogen-stripped CO-loaded methane is separated into nitrogen-contaminated carbon monoxide overheads vapor and methane-rich bottoms liquid in a methane-separation fractionation column in which both overheads cooling and bottoms reboil is indirectly provided by the CO recycle heat pump stream. Nitrogen is removed from the carbon monoxide overheads in a nitrogen/CO fractionation column to provide CO product bottoms liquid. Overheads cooling to the nitrogen/CO fractionation column is indirectly provided by expanded CO product bottoms liquid and bottom reboil is directly provided by the CO recycle heat pump stream.
EP-A-0676373 discloses a similar process for the recovery of carbon monoxide but in which hydrogen is separated from synthesis gas feed by partial condensation. The condensate is separated into nitrogen-contaminated carbon monoxide overheads vapor and methane-rich bottoms liquid in a methane-separation fractionation column. Nitrogen is removed from the carbon monoxide overheads in a nitrogen/CO fractionation column to provide CO product bottoms liquid. Partial condensation of overheads from at least one of said fractionation columns and bottoms reboil to the nitrogen/CO fractionation column are provided by a CO recycle heat pump stream. In one embodiment (FIG. 5), CO product bottoms liquid from the nitrogen/CO fractionation column is further distilled in an argon/CO fractionation column to provide argon-freed CO overheads vapor and an argon-enriched bottoms liquid. Bottoms reboil for the argon/CO fractionation column also is provided by the CO recycle heat pump stream.
The stated characterising feature of the process of EP-A-0676373 is reduction of energy consumption and plant capital cost by providing overheads condensation for only one of said separation columns and refluxing the other of said columns with liquid extracted at an intermediate location of the said column having overheads condensation. However, it does describe a process (FIG. 2) which does not have said reflux feature but partially condenses overheads of both the methane- and nitrogen-separation columns.
U.S. Pat. No. 5,592,831 discloses a process for recovering carbon monoxide from a feed containing at least hydrogen, carbon monoxide and methane. The feed is cooled and partially condensed and then scrubbed with liquid methane. Dissolved hydrogen in the resultant CO-loaded liquid methane stream is stripped and the hydrogen-stripped CO-loaded liquid methane stream is rectified into a CO-enriched vapor and a methane-enriched bottoms liquid. The characterizing feature of the process is that the liquid methane used to scrub the partially condensed feed contains at least 2 to 15 mol % CO. In practice, the scrubbing liquid is a major portion of the methane-enriched bottoms liquid from the rectification.
DE-A-19541339 discloses a process for removing nitrogen from synthesis gas in which the synthesis gas feed is partially condensed and hydrogen is removed from the condensed fraction in a stripping column to provide a hydrogen-freed CO-rich liquid. Nitrogen is separated from said CO-rich liquid in a nitrogen-separation fractionation column to provide a nitrogen-freed CO-rich bottoms liquid. Part of said nitrogen-freed CO-rich bottoms liquid is vaporized and both the vaporised and remaining (liquid) portions are fed to a methane-separation fractionation column to provide CO product overheads vapor and methane bottoms liquid. Optionally, additional CO is recovered from the hydrogen-rich vapor portion of said partial condensation of the synthesis gas feed by, for example, pressure swing adsorption or membrane separation and processing of the flush gas or membrane retentate.
Reboil to all three columns of DE-A-19541339 is provided by vaporizing a portion of the respective bottoms liquid and returning the vaporized portion to the relevant column. In one embodiment (FIG. 1), heat duty for the reboil of all three columns and condensation duty for reflux of the nitrogen-separation column is provided by a CO recycle heat pump stream, which also directly provides reflux to the methane-separation column. In remaining embodiments (FIGS. 2 & 3), heat duty for the reboil of all three columns and condensation duty for reflux of both the nitrogen- and methane-separation columns is provided by a (nitrogen) closed circuit heat pump stream.
A specified advantage of the process of DE-A-19541339 is the absence of a methane wash. In particular, it is stated that the successive nitrogen- and methane-separation fractionations avoid the use of a methane wash and thereby saves both capital and energy costs. However, in the absence of the optional recovery of CO from the hydrogen-rich vapor fraction of the synthesis gas feed, the CO yield of the process is only about 85%. The optional additional recovery of CO from the hydrogen-rich vapor fraction can increase the yield to about 97% but at the expense of additional capital and energy costs.
It is an object of the present invention to provide a more cost effective process for separating carbon monoxide from gaseous mixtures containing carbon monoxide, hydrogen, methane and nitrogen, especially those which also contain argon.